The present invention relates generally to improvements in electronic timepieces and more particularly to electronic timepieces having; means for reducing the power consumption of a stepping motor.
The present invention is best understood with reference to the prior art analogue type electronic wrist watch explained hereinafter.
The display mechanism of the conventional crystal watch of the analogue type now in use is constructed as shown in FIG. 1. The output of the timepiece motor comprised of a stator 1, a coil 7 and a rotor 6 is transmitted to different wheels 2, 3, 4 and 5 and a second hand, a minute hand, an hour hand as well as a calendar are driven by the wheels together with other wheels not shown.
The power consumption of a wrist watch is very small except for during a calendar switching operation, and a torque of 1.0 g-cm is sufficient for driving a second wheel in normal condition, while it is necessary to provide twice that torque for switching a calendar.
The time for switching a calendar is only six hours in twenty four hours, however a substantial amount of power for driving the watch movement is applied to the stepping motor during the calendar switching operation.
FIG. 2 shows a circuit construction of the conventional electronic watch. A signal of about 32 KHz, suitable for use as a time standard, is generated by an oscillating circuit 10 and is frequency-divided into a second signal by a frequency dividing circuit 11. The second signal is combined into the signal having either 7.8 msec or 2 sec period by a pulse combining circuit 12. To input terminals 15, 16 of drive inverters 13a and 13b is applied to a signal having the same pulse period and width but dephased by one second, so that an inverted pulse alternating every one second is applied to a coil 14. The rotor 6 magnetized in two poles begins to rotate in one direction. The coil current wave shape in this case is shown in FIG. 3.
In the meantime, the drive pulse width is the electronic watch according to the conventional art, i.e. 7.8 msec in the above mentioned embodiment, is designed in such a way that the factors such as coil resistance, number of turns, and size of the stepping motor are suitably selected so as to drive the stepping motor in a stable condition even when the load of the wheels weighs much, when the motor is placed in a magnetic field, when the internal resistance of the timepiece battery has been strikingly increased in a very low temperature, of when the battery voltage has been lowered because of exhaustion of the battery. Thus when a large torque is not necessary, this causes excess consumption of the battery.